1. Field of the Invention
This invention relates to a device and method for processing signals from a radiation detector with semiconductors. The invention is applicable more precisely to detectors such as gamma (.gamma.) radiation detectors that comprise a plurality of elementary detectors placed side by side along a surface or a volume of detection.
A device or a method of processing signals conforming to the invention can be used notably in the field of medical imaging for gamma cameras with CdTe or CdZnTe type detectors.
2. Discussion of the Background
FIG. 1 appended shows the principle of operation of an elementary detector with semiconductors of the CdTe type.
The detector in semiconductor 10 is equipped with two electrodes 12, 14 arranged respectively on two opposite faces. The electrodes 12 and 14 act both as polarization electrodes for the detector and as electrodes for reading the detection signals.
To provide the polarization for the detector, the electrodes 12 and 14 are connected respectively to a reference ground potential 16 and to a voltage source 18. The potential difference applied to the opposite faces of the semiconductor allows an electric field to be generated.
Hence, when a ray reaches the semiconductor material and creates an electron-hole pair, these electrical carriers do not recombine but, under the effect of the electric field are driven towards the electrodes 12, 14. An electrical signal is then tapped from these electrodes which is representative of the energy given up by the radiation to the semiconductor.
As FIG. 1 shows, the detection signal taken from electrode 14, is directed to means of processing the signal 24 after having been edited by an amplifier circuit 22.
In the text that follows, any interaction between a ray and the detector, during which the ray gives up to the material all or part of its energy, is referred to as an event.
In a detection head comprising a plurality of elementary detectors placed side by side, the geographic location of events is given by the co-ordinates of the detectors on a detection surface. Hence, an image of the source of radiation, gathered by the detection head can be established by suitable imaging means.
For information on this subject, reference can be made to documents (1), (2) and (3), the references of which are given at the end of this description.
In the case of the detection of gamma (.gamma.) radiation, two types of first order interactions can be distinguished.
In the rest of this text, the first type of interaction during which a gamma ray, called "an incident ray", on reaching the detector, gives up all its energy to the detector material, is referred to as a "photoelectric interaction".
On the other hand, a second type of interaction during which an incident ray only gives up part of its energy, is referred to as a "Compton interaction".
In the text that follows, the radiation coming from an observation target and reaching the detector is referred to as incident radiation. On the other hand, the radiation resulting from a Compton interaction during which the incident radiation only gives up a part of its energy, is referred to as "induced radiation". The induced radiation can also interact with the material and give up its energy to it. In this case, the total energy of the incident radiation is given up in several (generally two) interactions.
When electromagnetic radiation such as a gamma ray reaches a detection head formed by a plurality of elementary detectors placed side by side, three different cases of detection, represented in FIGS. 2A to 2C in very diagrammatic fashion, can be distinguished. For simplification purposes only the elementary detectors are shown in these Figures.
In the first case, illustrated in FIG. 2A, the interaction of a ray 30 with the semiconductor 10 of the detector is an interaction of the photoelectric type as described above. The incident ray 30 gives up all its energy in a single interaction given reference number 31. The detection signal obtained then corresponds to the total energy of the radiation.
In a second case, illustrated in FIG. 2B, the interaction is a Compton interaction. The incident ray 30 gives up a part of its energy during a first interaction labeled with reference number 31 and an induced ray 33 appears. In its turn, the induced ray gives up its energy in a second interaction which, in this example, is of the photo-electric type, labeled with reference number 35. It should be noted that the first and the second interaction both take place in the same elementary detector.
Hence, as these two interactions are quasi-simultaneous, the signal supplied to the detector terminals corresponds finally to the sum of the energies given up during the first and second interactions, that is to say to the energy of the incident radiation 30.
In a third case, illustrated in FIG. 2C, the energy of the incident radiation 30 is also given up during the two interactions 31 and 35. However, in contrast to FIG. 2B, the two interactions 31 and 35 have not taken place in the same detector but in two neighboring detectors 10 and 10a in the detection head.
The signal supplied by the first detector 10 in which the first interaction took place corresponds to the energy given up at the time of the first interaction, that is to say the energy of the incident ray 31 minus the energy of the induced ray. The signal supplied by the second detector 10a only corresponds to the energy of the induced ray, given up to the material during the second interaction. Finally, in this case, neither of the signals from the first or second detectors 10, 10a reflects the energy of the incident ray.
In medical imaging, generally a patient is injected with a radioactive isotope that emits gamma rays with a known and specified energy.
The gamma rays emitted in the patient can also interact with the tissue surrounding the organ of the patient being examined causing Compton type interactions of the type described.
Hence the radiation coming from the patient and reaching the detection head of a gamma ray camera not only includes the radiation from the isotope with known and specific energy but also the induced radiation or "Compton radiation" the energy of which is lower.
In order to prevent the induced or Compton radiation produced in the patient and considered to be parasitic, being taken into account, energy discrimination is carried out at the output from the detectors in such a way that only signals that correspond to the known energy of the radioactive isotope injected into the patient are retained. In other words only the so-called "useful" radiation is retained.
It can therefore be understood that the detection of a ray such as that described in the context of FIG. 2C, is particularly problematical.
In effect, when an incident ray, whose energy is equal to the specified energy that corresponds to the isotope injected into the patient, produces two interactions in two different detectors, the signal received from each detector corresponds to an energy lower than that of the incident radiation as indicated above. These signals are then removed by the energy discrimination operation previously described.
Hence the contribution of a "useful" ray is removed in error since it is taken to be a ray arising from a Compton type interaction within the patient.
Due to the fact that a dose of radioactive product injected into a patient must be relatively limited for health reasons, it may be understood that it is not appropriate to remove too large a number of useful incident rays. In effect as the data acquisition time to form a medical image cannot be too long, particularly for reasons of patient comfort, an over-restricted number of useful rays leads to a deterioration in the quality of the image that is finally obtained.
Therefore this invention proposes to resolve the problem of the loss of information associated with rays which interact in two separate elementary detectors in the detection head.